drm/i915: Fix VLV frame counter registers

Supposedly VLV uses the CTG+ style frame counter registers instead of
the old gen3/4 style. Add the magic offset to the correct registers.

We should already be taking the correct codepaths for
.get_vblank_counter() and .get_scanout_position().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>